Proteins are major targets of oxidative modification by many environmental toxicants. They lose function & structure, and must be proteolyticaly degraded or they will aggregate and form cross- linked cellular inclusion bodies. We have shown a major role for the Proteasome in detoxifying oxidized proteins. Now we find proteasome is under dynamic control, exchanging regulators and catalytic subunits, and exhibiting de novo synthesis in adaptation to mild, acute stress. Our Long-term Goals are to understand the mechanisms by which Proteasome contributes to basal and inducible oxidative stress resistance, the contributions of this detoxification system to human health, and how aging can compromise such resistance. Our Specific Aims are to test the following hypotheses: 1) Exposure of human HBE1 cells or primary human NHBE cells to hydrogen peroxide (H2O2), causes transient and reversible disassembly of 26S Proteasomes catalyzed by Ecm29, with stabilization of 19S regulators by HSP70. Dissociation of 26S Proteasomes prevents Nrf2 degradation, leading to Nrf2 accumulation, phosphorylation, and nuclear translocation; 2) Exposure of HBE1 human cells or primary human NHBE cells to a mild adaptive dose of H2O2, causes Nrf2 to bind to ARE (or EpRE) sequences of 20S Proteasome subunit genes and Pa28??? genes, and up-regulate their expression within 5-10 hours after H2O2 exposure. The Irf-1 and/or NF?B transcription factors simultaneously bind to upstream regions of the three Immunoproteasome genes, and up-regulate Immunoproteasome and Pa28?? expression; 3) Adaptation of C. elegans and D. melanogaster to H2O2 requires 20S Proteasome and Pa28? expression, via the skn-1 and cnc-C homologs of Nrf2; 4)The capacity to adapt to oxidative stress declines in senescent HBE1 and primary NHBE cells, in our model of hyperoxia-accelerated aging of non-dividing HBE1 and NHBE cells, and in NHBE cells from older human donors, partly due to diminished Nrf2 and Irf-1/NF?B signaling of Proteasome, Pa2???, and Immunoproteasome synthesis. H2O2-induced expression of Pa28? and Proteasome, through Nrf2 homologs skn-1 and cnc-C, also decays during aging of C. elegans and D. melanogaster, contributing to age-related decline in overall stress resistance and adaptability.